It is widely known in the art that the processing of crude oil and its various fractions has led to damage to piping and other associated equipment due to naphthenic acid corrosion. Generally speaking, naphthenic acid corrosion occurs when the crude being processed has a neutralization number or total acid number (TAN), expressed as the milligrams of potassium hydroxide required to neutralize the acids in a one gram sample, above 0.2. It is also known that naphthenic acid-containing hydrocarbon is at a temperature between about 200.degree. C. and 400.degree. C. (approximately 400.degree. F.-750.degree. F.), and also when fluid velocities are high or liquid impinges on process surfaces e.g. in transfer lines, return bends and restricted flow areas. Additional background on the problem of naphthenic acid corrosion in oil refineries can be found in Gutzeit, Materials Performance, pp 24-35, October, 1977; Piehl, NACE Corrosion 87 Meeting, Paper No. 196, Mar. 9-13, 1987; and Scattergood et al., NACE Corrosion 87 Meeting, Paper No. 197, Mar. 9-14, 1987.
Various approaches to controlling naphthenic acid corrosion have included neutralization and/or removal of naphthenic acids from the crude being processed; blending low acid number oils with corrosive high acid number oils to reduce the overall neutralization number; and the use of relatively expensive corrosion-resistant alloys in the construction of the piping and associated equipment. These attempts are generally disadvantageous in that they require additional processing and/or add substantial costs to treatment of the crude oil. Alternatively, various amine and amide based corrosion inhibitors are commercially available, but these are generally ineffective in the high temperature environment of naphthenic acid corrosion. Naphthenic acid corrosion is readily distinguished from conventional fouling problems such as coking and polymer deposition which can occur in ethylene cracking and other hydrocarbon processing reactions using petroleum based feedstocks. Naphthenic acid corrosion produces a characteristic grooving of the metal in contact with the corrosive stream. In contrast, coke deposits generally have corrosive effects due to carburization, erosion and metal dusting.
U.S. Pat. No. 3,531,394 to Koszman described the use of phosphorus and/or bismuth compounds in the cracking zone of petroleum steam furnaces to inhibit coke formation on the furnace tube walls.
U.S. Pat. No. 4,024,049 to Shell et al discloses compounds substantially as described and claimed herein for use as refinery antifoulants. While effective as antifoulant materials, materials of this type have not heretofore been used as corrosion inhibitors in the manner set forth herein. While this reference teaches the addition of thiophosphate esters such as those used in the subject invention to the incoming feed, due to the non-volatile nature of the ester materials they do not distill into the column to protect the column, the pumparound piping, or further process steps. I have found that by injecting the thiophosphate esters as taught herein, surprising activity is obtained in preventing the occurrence of naphthenic acid corrosion in distillation columns, pumparound piping, and associated equipment
U.S. Pat. No. 4,105,540 to Weinland describes phosphorus containing compounds as antifoulant additives in ethylene cracking furnaces. The phosphorus compounds employed are mono- and di-ester phosphate and phosphite compounds having at least one hydrogen moiety complexed with an amine.
U.S. Pat. No. 4,542,253 to Kaplan et al, described an improved method of reducing fouling and corrosion in ethylene cracking furnaces using petroleum feedstocks including at least 10 ppm of a water soluble mine complexed phosphate, phosphite, thiophosphate or thiophosphite ester compound, wherein the amine has a partition coefficient greater than 1.0 (equal solubility in both aqueous and hydrocarbon solvents).
U.S. Pat. No. 4,842,716 to Kaplan et al describes an improved method for reducing fouling and corrosion at least 10 ppm of a combination of a phosphorus antifoulant compound and a filming inhibitor. The phosphorus compound is a phosphate, phosphite, thiophosphate or thiophosphite ester compound. The filming inhibitor is an imidazoline compound.
U.S. Pat. No. 4,941,994 Zetmeisl et al discloses a naphthenic acid corrosion inhibitor comprising a dialkyl or trialkylphosphite in combination with an optional thiazoline.
Naphthenic Acid Corrosion in a Refinery Setting by Babaian-Kibala, Craig, Jr., Rusk, Blanchard, Rose, Uehlein, Quinter and Summers, Paper 631 of the 1993 NACE Annual Conference and Corrosion Show discloses the addition of certain phosphate esters to control corrosion in the high temperature areas of a distillation column by injection of the phosphate ester material into a draw tray and the packing above the heavy vacuum gas oil draw tray in a vacuum distillation tower. The injection of the phosphate ester in this manner reportedly lowered the corrosion rate in the tower.
While the art has suggested adding thio-phosphate materials of the type described herein to hydrocarbon distillation processes, the art has added such materials to the vessels containing the distilling petroleum fluid such as for example in the Weinland patent mentioned above. The non-volatile materials of this invention have accordingly not heretofore been applied to the surfaces of distillation columns, trays in distillation columns, pumparound piping, or the like which are susceptible to naphtheic acid corrosion. Accordingly, it would be very desirable to have available an enhanced naphthenic acid corrosion inhibitor for distillation columns and associated equipment including distillation trays and pumparound piping which is an effective naphthenic acid corrosion control additive.